I. Field of the Invention
This invention relates to diazotype photographic materials. In particular, this invention relates to diazotype materials of increased photographic speed, and to a method for preparing such materials.
Diazo films and the imaging process associated therewith are well known in the art of photography and photoprinting materials. The imaging process is based upon the photodecomposition of diazonium salts when the latter are exposed to light, and the reaction between diazonium salts and appropriate couplers to produce azo dyes under alkaline conditions. According to the process, a light-sensitive layer containing a diazonium salt and a coupler is exposed to a lighted image, and the diazonium compound in the light-struck areas decomposes. When the entire layer is thereafter subjected to alkaline treatment, the residual diazonium salt in the areas not exposed to light couples with the azo coupling component to form an azo dye image of the original pattern. The film portions exposed to light thus have a low optical density, whereas a permanent dye color is formed in the remaining portions by the coupling reaction.
Diazotype films are generally composed of an inert base support with a thin coating of the active components in a vehicle or binding material. The latter is applied in the form of a liquid emulsion and then allowed to solidify prior to image-wise exposure.
Unfortunately, the rate of photodecomposition for diazonium compounds is poor compared to the response rate of other photographic processes, notably the silver halide emulsion process. Furthermore, no chemical amplification technique is currently available to promote the photolysis of the diazonium salts. Since the rate of photolysis is proportional to the concentration of undecomposed diazonium compound present in the film, the rate decreases sharply after the initial exposure of the film to light. The exposure time thus required to decompose the last remaining amounts of diazonium compound is relatively long. For this reason, diazotype materials have a low printing speed, requiring long exposures even with proper light sources.
The printing speed of a diazotype film is dependent on several factors, including the absorption coefficient of the diazonium compound with respect to the wavelength of the light to which the film is exposed, the length of the optical path through the diazotype film, and the concentration of the diazonium salt in the film. It is, accordingly, an object of the present invention to enhance the sensitivity of diazotype materials by increasing the optical path length through a film of given thickness.
Other objects will be apparent from the following description.
II. Description of the Prior Art
Diazotype films and methods of preparation thereof are generally disclosed in Peterson, U.S. Pat. No. 2,501,874 (Mar. 28, 1950), Halperin, et al., U.S. Pat. No. 3,301,679 (Jan. 31, 1967), van der Grinten, The Photographic Journal, 92B:43-48 (1952), and Landau, et al., J. Photog. Sci., 13:144-151 (1965).
A similar problem of slow printing speeds exists in vesicular photographic materials. Various means for increasing the speed of such materials are disclosed in Schoen, et al., U.S. Pat. No. 2,908,572 (Oct. 13, 1959), Baril, et al., U.S. Pat. No. 3,149,971 (Sept. 22, 1964), and Nishino, U.S. Pat. No. 3,841,874 (Oct. 15, 1974). The chemistry of vesicular photography, however, is radically different from that of diazotype photography. In vesicular photography, a compound which undergoes photolysis to yield gaseous products is dispersed in a polymeric vehicle. Development of the image is accomplished by the application of heat, and image formation is accomplished by light scattering rather than light absorption. Both the chemistry and the physical interactions are entirely distinct from those of the diazotype process.